Field of the Invention
The present invention relates to a multiple stage automatic transmission in which a main transmission is connected with a sub-transmission in tandem to increase the number of shift stages of a system as a whole.
Description of the Prior Art
Generally, an automatic transmission for an automotive vehicle is provided with a torque converter and a shift gear mechanism wherein the torque converter amplifies an engine output torque and transmits it to a turbine shaft and wherein the shift gear mechanism shifts the torque transmitted to the turbine shaft and transmits it for drive wheels. The shift gear mechanism is, generally, provided with a planetary gear mechanism having a sun gear, ring gear, pinion gear and carrier in combination with a plurality of frictional elements, such as a clutch, brake which are selectively actuated to be engaged and disengaged. Thus, a hydraulic control mechanism is employed to switch on-off patterns of the frictional elements to thereby establish a desired shift stage automatically.
In the automatic transmission, as the number of the shift stages is increased, torque transmission characteristics can be broadly selected in accordance with a road condition or running condition of the vehicle so that a running performance and fuel consumption efficiency can be improved. It is limited to provide such broad selectivity of the torque transmission characteristics when a single shift gear mechanism is used. The single shift gear mechanism can provide at most four different shift stages for forward movement.
In view of this, it has been proposed that a transmission system provided with two shift gear mechanisms (main transmission and sub-transmission) which are combined in tandem to increase the number of shift gear stages. For example, Japanese Patent Public Disclosure No. 51-127968, laid open to the public in 1976, discloses such type of transmission system.
U.S. Pat. No. 5,109,731 discloses an automatic transmission in which a shift operation is made in the sub-transmission in response to a shift operation of the main transmission.
If the main transmission is provided with three shift stages and sub-transmission is provided with two shift stages for forward movement, the combined transmission can provide six shift stages. If five shift stages are actually required, one shift stage as provided by a change of combination of the main and sub-transmissions may be omitted out of six combinations.
When so called schedule up-shift in which an up-shift is made under a constant throttle opening is required, it is necessary for the sub-transmission to make a shift change by two steps. In detail, while shift changes are made in the main transmission among low (Lo), middle (Mid) and high (Hi) speed stages, shift changes are made in the sub-transmission between low and high speed stages for two stages among the three stages of the main transmission. The amount of the torque transmitted to the sub-transmission is provided by multiplying an input torque for the whole transmission or the main transmission by a gear ratio of the main transmission.
Thus, as a torque difference in the main transmission from one to the other is increased, a control band for a frictional element such as clutch and brake in the sub-transmission involved in the shift operation is increased. This means that the control band is widely changed depending on the shift operation. The frictional element is, usually, controlled by a hydraulic pressure. The maximum capacity of the frictional element needed can be determined by a physical load condition which is applied to the frictional element. If the control band is widely changed within the range of the capacity of the frictional element, the hydraulic pressure for controlling the frictional element is needed to be properly controlled to make a shift operation.
Conventionally, the frictional element is controlled by means of a single piston in a manner that a predetermined hydraulic pressure is introduced into an apply chamber to move the piston to engage the frictional element. In this control, as the control band required is increased, in other words, if the difference in engaging force which corresponds to the torque difference between the two frictional elements for transmitting the torque therethrough is increased, the change of the hydraulic pressure for applying the frictional element is increased. This makes the control of the hydraulic pressure difficult.
Specifically, if the sub-transmission is switch between the high and low speed stages in the case where the main-transmission is switched between the low and high speed stages (not consecutive stages), the torque difference in the main transmission involved in the shift operation is tremendous to cause a tremendous torque difference in the sub-transmission as well. This makes the hydraulic control for the frictional element of the sub-transmission more difficult.